The present invention is generally related to an apparatus and a method for curing composite resin/fiber mixtures using microwave wavelength electromagnetic radiation to ensure uniformity of curing temperature.
The pultrusion process is a conventional process in which strands of fibrous material, such as glass fiber or graphite fiber, are impregnated in a polymer resin bath and pulled through one or more dies or preformers prior to curing the resultant shape in a temperature controlled environment . The pultrusion process has been particularly useful in the production of thin tape or ribbons or in the production of rod-shaped masses exhibiting relatively small radius. Conventional heated dies have performed satisfactorily in manufacturing these thin tapes and small radius rods even though the pulling speed is generally less than desirable. However, for thick tape or large diameter rod shaping, nonuniform cure can result in warp or delamination of the final product. Because of the poor thermal conductivity of the polymer resin/fiber composite systems, nonuniform heating and uncontrollable chemical reactions appear to result and to thereby effect a reduction in the strength of the cured product. The most important technical aspect of the whole pultrusion process is the thermal control of cure or hardening during the pulling operation. This is particularly true in the curing of thick sections (over 3 centimeters) in the pultrusion process. Such thick sections have exhibited a tendency for internal cracking and delamination Conventionally heated dies have not proven to be a satisfactory solution to this problem which appears to be due to the low thermal conductivity of the resin/fiber composite being cured. The heat of reaction released locally is therefore not quickly distributed throughout the workpiece and, accordingly, large temperature gradients can exist. This results in localized heating which further accelerates the local reaction and worsens the curing problem. It is therefore desirable to obtain a system for heating and curing the entire thick composite section simultaneously throughout the mass with the lowest possible heat input.